Seismic prospecting system



Oct. 27, 1964 L. H. JOHNSON SEISMIC PROSPECTING SYSTEM 5 Sheets-Sheet 1 Filed May 15, 1955 Can/roller INVENTOR BY K Afrakfwzy- .LENARD H (bl-INSON,

Oct. 27, 1964 1.. H. JOHNSON SEISMIC PROSPECTING SYSTEM 5 Sheets-Sheet 2 Filed May 15, 1953 INVENTOR (Ll-M7501; v

7'TO QNE V BY LE 20 2 WNR L. H. JOHNSON SEISMIC PROSPECTING SYSTEM Oct. 27, 1964 5 sheets vsheet 5 Filed May 15, 1953 I I I I I I WWW wr W% w FIG. 6.

United States Patent 3,154,760 SEESMKI PRGSPECTENG SYSTEM Lenard H. Johnson, Pasadena, Calif, assignor, by meme assignments, to United Geophysical Corporation, Pasadena, Qaiih, a corporation of California Fiied May f5, 1953, S61. No. 355,374 3 (iiaims. (Ci. M l-15.5)

This invention relates to seismic prospecting and more particularly to methods for recording and reproducing seismic waves.

In reflection seismic prospecting as generally practiced, a charge of explosive is detonated at a shothole, causing seismic Waves to travel outwardly therefrom in all directions. Some of these waves are refracter and reflected by underlying formations, thereby being returned to the surface of the earth where they are received by seismometers and converted into corresponding electrical waves. In eifect, a train of seismic waves is received at each seismometer and is converted into a train of corresponding electrical waves.

As is well known, it is often desirable to record seismic waves in limited frequency bands in order to eliminate undesirable seismic waves or extraneous seismic disturbances and to emphasize certain seismic waves which are valuable in determining the structure of the subsurface formations. Sometimes only one shot is fired in a shothole because the shothole collapses after the first shot. It is therefore highly desirable to produce a record of all seismic Waves received from thatone shot and to subsequently reproduce from that single record seismic waves in limited frequency bands. For this and other reasons, it is desirable to record seismic waves in such a form that they can be reproduced and filtered in any desired way.

In one method of recording seismic waves, the waves are recorded on a continuous or endless record strip, such as a magnetic tape, in the form of a closed loop. Such a loop is formed by splicing together the ends of an elongated strip. Even though the original strip may be uniform and homogeneous throughout its length, the splice so formed constitutes a local inhomogeneity that may seriously interfere with the recording.

In recording waves on such an endless loop, it is desirable to avoid any overlapping of the tracks with the splice. If such overlapping occurs, the record may be lost in an important section thereof. Furthermore, the splice may so react with the recording element as to disable it in some fashion, thereby preventing the making of accurate records for a substantial period. More specifically, when a magnetic tape is employed, it is desirable to use magnetic heads for recording. Such magnetic heads are provided with small gaps adjacent the record strip. For example, the gaps may be only about 0.0005 inch wide. The gap formed by the splice may be about 0.10 inch wide or there may be an overlap. Narrow gaps are employed in the magnetic heads in order that the lengths of the magnetic tapes will not become unduly great and yet no matter whether the splice is in the form of a butt joint or lap joint, it is likely to interfere with the recording. One reason for this is that it may be difiicult to maintain the magnetic tape sufiiciently close to the magnetic heads as the splice passes them to produce a satisfactory record for a substantial period after the splice has passed the magnetic head.

One object of the invention is to overcome the foregoing difficulties.

; Another object of the invention is to provide an improved system for recording seismic waves that is 0011- venient to use' and positive in operation.

Another object of the invention is to provide an im- 3,154,760 Patented Get. 27, 1964 proved system for recording reproducible records of seismic waves over a broad band of frequencies.

Another object of the invention is to provide an improved system for reproducing seismic waves from a reproducible record.

Another object of the invention is to provide an arrangement for automatically operating: various control elements of a seismic wave recording system in timed relationship with the passage of a predetermined portion of the record strip past the recording elements.

Another object of the invention is to provide a system which indicates when a splice in an endless record strip passes the recording elements.

Another object is to provide an arnangement for operating the recorder only during a single passage of a record strip past the recording elements without the record overlapping a splice in the record strip.

The foregoing and other objects of the invention will be apparent from a study of the following specification, read in connection with the accompanying drawings, in which:

FIG/1 is a schematic diagram of a recording and reproducing system of this invention;

FIGS. 2a and 2b are second diagrams of the system illustrating various parts thereof in detail;

FIG. 3 is a schematic diagram of a recording channel;

FIG. 4 is a schematic diagram of a reproducing systern;

FIG. 5 is a composite diagram showing how FIGS. 2a and 2b are assembled; and

FIG. 6 is a schematic diagram of a modification of the invention.

A seisrnograph recording and reproducing system embodying features of this invention includes three main parts as shown in FIG. 1, namely, a seismic wave recorder 100, a seismic wave reproducer 2th and a control unit 3%. According to this invention, seismic waves received at a plurality of spaced points in the earth are recorded as reproducible tracks on a continuous record belt M and are subsequently reproduced therefrom and rereoorded to form oscillograph traces by means of a multiple-element osciilograph O, the same transducing devices D being employed for recording the tracks on the record M and for reproducing them so as to form the traces.

In this specification the invention is explained with reference to the reflection seismic prospecting system of the type described and claimed in cope-nding patent application Serial No. 319,969, filed by Raymond A. Peterson, November 12, 1952. As shown in FIG. 1 hereof a charge of explosive E is located at the bottom of a shot hoie SH and a series of seisrnometers in the form of hydrophones S 8 are arranged in a vertical line in a receiver hole RH some distance from the shot hole SH. An uphole seismometer S is mounted at the top of the shot hole SH. In using such a system the charge of explosive E is detonated by operation of blaster B. In the present invention the seismic waves thereafter received at the various seismo-meters S S S are recorded on a reproducible record and are then subsequently reproduced.

The seismic wave recorder comprises a plurality of channels H H having their respective inputs connected to seisrnometers S S and their respective outputs connectable to the respective transducing heads D D The recorder lilii also includes auxiliary channel H which has an input connected to a mixing unit 129 and its output connectable to a transduc-ing head D The seismic wave reproducer 2% comprises a coresponding series of reproducing channels h h ducing devices D D having their respective inputs conneotable to the transducing heads D D and their outputs connected to recording elements g g in the form of galvanometers of the multiple-element oscillograph O. The

reproducer 200 also includes an auxiliary reproducing channel 71 having an input conneetable to the auxiliary transducing device D and its output connected to an auxiliary element g in the form of a galvanometer of the oscillograph O.

The record belt M is mounted on a driving drum or 'wheel d and a driven or idler drum or wheel 3. The

driving drum (I is arranged to be driven continuously divided into a series of parallel longitudinal zones which form tracks Z0; Z z which travel past the trans- D as the belt is driven by the motor m in the direction indicated by the arrows The transducing devices D D D are mounted in .a transducing area Z adjacent the surface of the record belt. The transdu-cing devices may be arranged along a straight line transverse to the length of the belt, or they may be arranged in echelon, or in some other 'desired, pattern according to the needs of the operator.

However, for simplicity, it is assumed hereinafter that 'the transducing heads are arranged along a straight line transverse to the path 12.

In the embodiment of the invention specifically described herein, the record belt M is inthe form of a strip which is uniformly coated externally with a ma- 3 terial which is adapted to retain or remember magnetic fields impressed thereon. Such a record belt is known as a magnetic tape. Also in this embodiment of the invention, the transducing heads D D D are in the form of magnetic heads which are adapted to impress magnetic fields upon such a coating and which are also adapted to detect any residual magnetism retained by such coating.

In order to form the endless record belt M, the ends of a strip of record material are spliced together, either by means of adhesive paper, or otherwise. Either a butt joint or a lap joint may be formed at the splice P. In either event, when the splice P passes the transducing devices D, the splice is likely to interact with the transdncing devices D D D either mechan-icflly or magnetically in such a way as to disturb the recording or reproduction of the waves.

The record belt M is provided with a marker in the form of perforation p which is employed tooperate the control unit 300 in such a way as to prevent such of the amplifiers A comprises a variable attenuator V,

a first amplifier section A and a second amplifier A.

For convenience hereinafter the attenuators V are sometimes considered as part of the first amplifier sections A, especially when reference is made to the gain of these amplifier sections. The amplifiers A have broad-band characteristics, being adapted to amplify waves uniformly throughout a wide band of frequencies of interest in seismic prospecting such as a band extending from 5 'c.p.s. to 300 c.p.s. -'A relay R forming part of the control unit 390 is connected between the output of each 'of the channels H and the corresponding magnetic head In this application, the subscripts 0, 1 6 are applied to identical parts in the different channels and the subscripts are frequently omitted when the description applies equally to identical parts in the respective channels. Sometimes though subscripts are also employed to distinguish different similar parts all of which are associated with all of the channels.

The control unit Silt) is provided with a plurality of channel relays R R which are adapted to connect the magnetic heads D D selectively to the outputs of the respective recording channels H H or to the inputs of their respective reproducing channels ho h6- When the relays R R are operated, the magnetic heads D D are connected to the outputs of the recording channels H H and when the relays 7 R .arelrestored the.maguetic heads. areconnected to the respective reproducing channels h h As shown in FIG. 1 and FIG. 3 the seismic-wave recorder 1% includes an expander 110 for controlling the attenuation produced by the respective attenuators V V as a predetermined function of time. The seismic-wave recorder also includes a mixer 12% which is adapted to supply to the input of the auxiliary recording channel H various signals including a time break produced by a blaster B, a first break FB produced by an uphole seismometer S and a timing signal TS supplied from the oscillator 314).

The attenuators V are of a type which attenuate waves transmitted therethrough to a degree that varies inversely as the value of a voltage applied thereto, the degree of attenuation decreasing as the applied voltage increases. The expander is of a type which is adapted to provide the desired voltage that increases as a predetermined function of time, thereby gradually reducing the attenuation produced by the attenuators V. pander increases the gain of each of the first amplifier sections A" as a predetermined function of time.

Each of the second amplifier sections A" is part of an AVC loop. Such an amplifier section tends to limit the amplitude of the signal at its output to a predetermined AVC level, but since such an AVC loop'possesses both an attack time and a recovery time, a short-time variation in the amplitude of the signal applied to the input thereof causes corresponding variations at the output thereof. Since this makes it possible to record seismic waves rather faithfully over a period of several cycles or wave lengths, the waves recorded on the tracks aid in the identification and correlation of seismic waves reflected from various portions of a subsurface stratum to the seismometers S S that are connected to the inputs of the various channels H 5 Each of the frequency-modulated generators G is o a type which generates at its output a square wave that is of constant amplitude but of variable frequency. The frequency-modulated generator G produces at its output a wave having a frequency that deviates from a reference frequency by an, amount A proportional to the amplitude of the voltage applied to its input. As shown in FIG. 3, the frequency-modulated generator G employed is in the form of a flip-flop circuit N which produces square waves at a frequency that deviates from a reference frequency by an amount proportional to the voltage impressed on its grid circuits. The voltage in this case is supplied from the output of the corresponding recorder amplifier A. An isolation network comprising an amplifier A having a cathode follower in the output stage prevents interaction between the magnetic head and the corresponding modulator G when relays are being switched. The output impedance, of the cathode follower stage of amplifier A"" is small compared with the impedance of the-associated magnetic head D. The amplitude of the pulses impressed upon "the head D is suificient to saturate the: record magnetically even though it may have beenpreviously saturated in the opposite direction. In practice, according to thefpresient invention, the amplitude of the output of the frequency-modu- In effect, the exlated generator is sutliciently'high to saturate the tracks produced on the magnetic record M in opposite directions in alternate half cycles.

When the outputs of the recording channels H H are connected to the magnetic heads D D waves are recorded along the tracks formed in the various zones z Z6. The waves are of substantially constant amplitude in view of the saturation of the magnetic material that is on the surface of the tape. If no signals are being applied to the inputs of the respective channels H H waves of a constant frequency equal to the reference frequency of the generators G G are recorded on the tracks. In practice, the frequency-modulated generators are designed to operate at some predetermined frequency, say about five thousand cycles per second, and the generator is adapted to vary in frequency by as much as ten percent when the amplitude at the output of the corresponding channel H reaches its AVC level. When signals are applied to the inputs of the channels H H the spacing between successive magnetized areas along the tracks is inversely proportional to the frequencies applied thereto.

The reproducing channels h h comprise demodulators I i and amplifiers A A If the magnetic heads D D are connected to the inputs of the reproducer channels h h then when the tape travels past the heads D D the signals on the tracks z Z6 cause pulses to be applied to the inputs of the reproducing channels h h The frequency of the pulses applied at any time equals the frequency produced by the cnerator G in the recording of the corresponding track.

Each demodulator I is of a kind which produces at its output voltages proportional to the deviations in the frequency applied thereto from the reference frequency. These voltages are then passed through the amplifiers A A of the reproducing channels and the amplified voltages are applied to the galvanometers g g of the multiple-element oscillograph. Thus, the reproducer Ztltl is adapted to convert variable fre quency magnetic tracks recorded on the magnetic tape M by the recorder see into variable amplitude oscillograph traces.

More particularly as shown in FIG. 4, each of the demodulators 3 comprises an input amplifier 21!), an amplitude clipper 212, a dilferentiator 214-, a pulse rate measuring device 216, a low-pass filter 21%, and an output amplifier 226, all connected in the order named. The input impedance of the input amplifier 21d is high compared to the impedance of the magnetic head to which it is connectable. The purpose of this amplifier is simply to amplify the pulses picked up by the head to a high degree. These pulses are then supplied to the clip or 212 where their peaks are cut oil, thereby producing substantially square waves. The difrerentiator 214 is in the form of a differentiating network which responds to the rate of change of voltage supplied to its input, producing positive or negative pips at its output according'to whether the change of voltage applied to its input is in one direction or the other. The frequency, or repetition rate of the pips is the same as the frequency of the s uare waves applied to the input of the demodulator and, hence, proportional to the frequency of the recorded waves. The pulse rate meter 216 may be or" any suitable type that produces at its output a voltage proportional to the frequency of the pulses applied to its input. Such a rate meter may comprise a thyratron which is ignited each time a pip is applied to its grid and which is connectedto produce a specific amount'of charge in an output circuit each time it is fired. Consequently, the wave appearing at the output of the pulse rate measuring device includes a component proportional to the voltages that were originally employed to modulate the corresponding frequency-modulated generator G with saw-tooth ripples thereonof the same frequency as the frequency of the pulses applied to its input. The frequency of the saw-tooth ripples superposed thereon is, however, very high, being equal to the frequency of the pulses.

The low-pass filter 218 has a cut-oil between the highest seismic wave frequencies of interest and the frequency of the lowest frequency Waves that are produced by the frequency modulated generator G. Thus, for example, the low-pass filter 218 may have a cut-elf at about five hundred cycles per second. Consequently, when the output of the pulse rate meter 216 is passed through the filter 218, a wave is produced that is a replica of the signal that modulated the corresponding generator G. This wave then contains all the components of the seismic waves passed through the corresponding amplifiers A.

The amplifiers A' contain adjustable filters for selectively amplifying components of the waves impressed on their inputs. These amplifiers may also be of the AVG type in order to limit the amplitude of the traces produced in the oscillograph O to an amplitude range suitable for study of both the relatively large and the relatively small waves.

According to the present invention, the relays R R are so operated that a record is taken only during a period when the splice P is outside the 'transducing area. Furthermore, in accordance with the present invention, the expander lltl is operated automatically to vary the gain of the first amplifier sections A of the various channels H according to a desired function of time during the period when the tracks are being recorded on the record belt M. Also, according to this invention, the mixer is so operated as to impress upon the input of the auxiliary channel H signals which permit recording of the time break and the uphole time and a timer track which aids in determining the exact instants of arrival of seismic waves at the various seismometers S 5 Also, in accordance with the present invention, a common generator 31ft is employed to drive the synchronous motors m and m and to produce the timing lines on the reproducible record.

As shown in FIGS. 1 and 2a, a sequence controller 326 is actuated by the marker p to indicate when the splice P is in the transducing zone Z and to render the relays R R operative to record seismic waves on the magnetic tape M only during a time interval when the splice P is outside the transdncing area. A detector element in the form of a switch has a pair of contacts li normally urged toward each other. The contacts are located on opposite sides or" the magnetic tape M at a position adiacent the transducing zone Z. The distance between the detector element and the transducing zone Z is about equal to the distance between the marker p and the splice P.

Furthermore, the detector element is located in such a direction from the transducing zone Z that the contacts K close when the splice P approaches or enters the transducing zone 2. Thus, with this arrangement, in each revolution of the belt, the perforation p enters the gap between the contacts K when the splice P is in such a position as to disturb the making or reproduction of a record.

Each time the contacts K close, an electrical pulse is applied to the input of the sequence controller 32%).

In order to provide the desired indications, controller 329 comprises three sequence relays R R and R as shown in FIG. 2a. In order to distinguish between contacts of the various relays, the contacts are specified hereinafter by the symbol K(a, b) where is the subscript of the corresponding relay R, with which the contacts are associated and b is an ordinal number employed to distinguish one pair of contacts of a relay from another in the same relay. Three thyratron tubes Y Y and Y are associated, respectively, with the three relays R R R of the sequence controller 3249. The thyratrons are normally rendered inoperative by a C bias applied to their grids from a suitable source of pow-er C.

i The first sequence relay R comprises normally open sticking contacts KC), 1), normally closed contacts KG,

and the like through terminal B ,2), normally open contacts K(7, 3) and two sets of normally open contacts K(7, 4) and normally open contacts K(7, 5). Voltage is supplied through solenoid winding W of relay R to the anode of the first sequence thyratron Y The anode is also connected to a sticking circuit including contacts K(7, 1) and a yellow CON- TAC light L The second sequence relay R comprises normally open sticking contacts K(8, 1), normally closed contacts K(8, 2), normally open contacts K(8, 3) and normally open contacts K(8, 4). Voltage is supplied through solenoid winding W of relay R to the anode of the second sequence thyratron Y The anode is also connected to a sticking circuit including contacts K(8, 1) and a red FIRE light L The third sequence relay R comprises normally open, sticking contacts K(9, 1) and normally closed contacts K(9, 2) and contacts K(9, 3). Voltage is supplied through solenoid winding W of the relay R to the anode of the third sequence thyratron Y The anode is also connected to a sticking circuit comprising contacts K(9, 1) and a blue STOP light L Each of the channel relays R associated with a magnetic head D is provided with a pair of normally closed contacts K(a, 1) and a pair of normally open contacts K(a, 2). The corresponding magnetic heads D,, are arranged to be connected to the input of the corresponding reproducing channel H, when the first pair of contacts K(a, 1) are closed, that is, when the corresponding channel relay R,, is restored; and to the outputs of the corresponding recording channels H when the second pair of contacts K(a, 2) are closed, that is, when the corresponding channel relay R is operated.

Various parts of the apparatus are energized by closure of a power switch E More particularly, when the power switch E is closed, the C- bias is applied to the various thyratrons Y Y Y and Y Also, when the power switch E is closed a high voltage of about one hundred or two hundred volts is applied to the various amplifiers A low B+ voltage of about twenty-four volts is applied to the detector contacts K, through terminal B and is available to energize the thyratrons Y Y and Y through a ready switch E when closed. A record-reproducer switch E is arranged between the output of sequence controller 320 and the channel relays R R The mixer 120 comprises a relay R having a thyratron Y associated therewith. When the power switch E is closed, B+ voltage is applied from voltage terminal B to the anode of the thyratron Y through the winding W of relay R Relay R comprises normally open contacts K(10, 1), a pair of normally closed contacts K(10, 2), a pair of normally open contacts K(10, 3),

normally closed contacts K(10, 4), normally closed contacts K(10, and normally open contacts K(10, 6).

' The normally closed contacts K(10, 1) are included in a sticking circuit, between the'anode of thyratron Y .and ground.

The pair of normally closed contacts K(10, 2) is connected to a pair of potentiometers p and p The output from a blaster B is applied to potentiometer p and the output from the uphole seismometer S5 is applied to the second potentiometer p With this arrangement, any desired fractions of the signals from the blaster and from input of the auxiliary channel H The normally open contacts K(7, 4) are arranged in the line between the B and B terminals and the input to the expander which includes series-connected control. resistors 112 and 114. Normally closed contacts K(10, 4) are connected across one control resistor 112.

One of control resistors 112 is arranged to be shortcincuited initially and subsequently to be unshorted. When in the shorted condition a relatively low voltage is applied to the attenuators V and the efiective gains of the first amplifier sections A are relatively low, but when the control resistor 112 is unshorted, the voltage applied to the attenuators V gradually increases, thereby gradually increasingthe effective gains of the first amplifier sections A as a predetermined function of time. As is well known, the rate of increase of gain depends upon the time constants of the resistance capacitance networks 116 in the expander 119 and the initial and ultimate values of the gain depend upon the values of the control resistors 112 and 114.

When a record is to be made, the power switch E and the motor switch E and the record-reproducer switch B are closed. All of the amplifiers A of the recorder are energized, but the frequency modulated generators G circuit completed through contacts K(10, 5) energizes a, green light L indicating the restored condition of relay R At this time, no voltage is applied to the expander 110by virtue of the fact that contacts K(7, 4) are open.

-Also, at this time, all of the magnetic heads D D are connected to the inputs of the corresponding reproducing channels h k through the corresponding contacts K(t), 1) K(ti, 1). It should be noted that when the apparatus is ready, no power is applied to the channel relays R the switch E and the contacts K(9, 3), and K(8, 4) because at this time the latter contacts K(8, 4) are in their restored condition in which they are open.

While the apparatus is in a ready condition as described above, the magnetic tape M is travelling continuously along its path 12. It will be noted at this point that the ready switch E may be opened at any time to remove power from the circuits including all channel relays R R and the controller relays R R and the relay R and that as will appear later, when the ready switch E is open, no record can be made on the magnetic tape.

As indicated above, when the operator is ready to proceed with the making of a record on the magnetic tape M he closes the ready switch E thus, energizing, or applying power to, the thyratrons Y Y Thereafter, each time the perforation p enters the; gap between the contacts K these contacts close, impressing a sharp positive pulse upon the input of the sequence controller 320.

When the detector contacts K close the first time, a positive voltage pulse is applied to the grid circuit of the first thyratron Y igniting it, thereby operating the first control relay R When the relay R operates, the sticking contacts K(7, 1) close, thereby-applying power to the yellow Contact light L At the same time contacts K(7, 4) close, applying power from the terminals B and B to the expander 110. As a result of the closure of the contacts K(7, 4) the various'condensers in the expander 110 become charged to their initial "voltage during the subsequent revolution of the magnetic tape M. Thus,

during this revolution of the magnetic tape M, the gains of the first amplifier sections A are brought to low values, thus preparing the recording channels H H; to re- R through the circuit including 9 ceive and amplify first breaks without excessive overloading of the amplifiers in the channels.

Also, when the relay R operates, contacts K('7, 2) open and contacts 1((7, 3) close, disconnecting the input of the sequence controller 320 from the grid circuit of the first sequence thyratron Y and connecting the input to the grid circuit of the second sequence thyratron Y through the normally closed contacts K(8, 2). As a result, the second sequence thyratron Y and the second sequence relay R are prepared for operation.

Also when the first sequence relay R operates, contacts K(7, close energizing the frequency modulated generators G. As a result, these generators become stabilized at a reference frequency during the next revolution of the tape M.

The next time that the perforation p enters the gap between the detector contacts K a second pulse is applied to the input of the sequence controller 329. This pulse is applied through the contacts K(7, 3) which are now closed and the normally closed contact 1((8, 2) to the grid circuit of the second sequence thyratron Y causing it to ignite and operate the second sequence relay R When relay R operates, sticking contact K(S, ll) closes, igniting the red FIRE light L At this same time contacts K(tl, 4) close, thereby operating all the channel relays R R When the channel relays R R operate, contacts 1((0, l) 1((6, i) open and contacts KG 2) K(6, 2) close, thereby disconnecting the magnetic heads D D from the inputs of the reproducing channels h them to the outputs of the recording channels H H Also, when the second sequence relay R operates, contacts 1((8, 2) open and contacts 14(8, 3) close, thereby connecting the grid circuit of the third sequence thyratron Y to the input of the sequence controller 32% thereby preparing both the thrid sequence thyratron Y and the third sequence relay R for operation. At the time that the ready light L is illuminated, the operator of the recording truck signals a shooter at the blaster B to detonate the charge of explosive E. In practice the operator delays giving the signal for a short time such as by counting one, two, three, FIRE.

When the blaster B is operated, an electric signal is applied to the input of the auxiliary recording channel H through the contacts KGW, 2) modulating the output of the frequency modulated generator G thereby energizing the corresponding transducing head D producing a record on the track in the zone z As is well known, when the charge of explosive E is detonated, seismic waves travel outwardly in all directions therefrom. Some of these seismic waves travel directly to the uphole seismometer S and others by direct and refracted paths to the remaining seismometers S S Subsequently, Waves retracted and reflected from subsurface formations are returned toward the surface of the earth in the form of wave trains which reach the various seismometers S S Waves reflected from a common horizon may arrive successively at the respective seismometers S S The relative times of arrival of the waves are measured and employed to determine the structure of the underlying formations. In any seismic prospecting system, some means is provided for recording the waves received at the seismometers together with a timing trace and the resultant record is examined to determine the relative times of arrival of the waves in question.

When the seismic waves arrive at the uphole seismometer S an electric wave generated by the seismorneter is applied to the input of the auxiliary channel H through the contacts (Kit), 2), again modulating the output of the frequency modulated generator G and producing a corresponding indication or record on the track in the zone z Either at the time that the blaster B is operated or at the time that a seismic wave first arrives at the uphole seismometer S as determined by the setting of a trip seh and connecting a red indicator light L lector switch E an electric wave is amplified by an amplifier 122 and applied to the grid circuit of the thyratron Y causing it to ignite and operate the relay R When the relay R operates, sticking contacts Kfltl, 1) close. At the same time contacts F410, 2) open and contacts K(ltl, 3) close disconnecting the input of the auxiliary recording channel H from the potentiometers p and p and connecting it to the output from the 50 cycle oscillator 3H1. Thereafter, the 50 cycle signal from the oscillator Sill is amplified by the auxiliary channel H thus modulating the output of the frequency modulated generator G at 50 c.p.s., thereby producing on the track in the zone z a timing track record which may be employed to determine the relative times of arrival of waves at the various seismometers S S The selector switch E has other contacts (not shown) allowing the selector switch E to be set to ignite the thyratron Y when a seismic wave is received by any one of the other seis mometers S S or a separate seisrnometer placed at any desired position.

It will be noted that if the switch E is set to prepare thyratron Y and relay R for operation by the signal from the blaster B, no record is made of Waves arriving at the uphole seisrnorneter S. Of course, if no uphole seismometer S is employed, the potentiometer p and the circuits leading to the uphole seismometer S may be dispensed with.

In either event, since the motor M is driven at a constant speed, the instant of detonation of the charge may be ascertained by extrapolating the indications on the timer trace to the time break.

Also, at the time that the relay R operates, contacts Kilt, 4) open, thereby increasing the voltage to which resistor capacitor networks 116 are ultimately charged. Thereafter, the effective gains of the input amplifier sections A, A inc ease as a predetermined function of time. it will be noted that the expansion of the gain starts at the time that the blaster B is fired or at the time that waves are first received at the uphole seismorneter S depending upon the setting of the trip selector switch Also, at the time that the relay R operates, contacts high, 5) open and contacts KCW, 5) close, thereby deenergizing the green indicator light L and energizing the In this way an indication is produced that the timer track is being recorded and the operation of the expander has been initiated.

It will be noted that the channel relays R R are connected to the out uts of the channel amplifiers A A only after the splice P has passed the transducing zone Z and that it is only then that the transducing heads D D are prepared for making records of the seismic waves received at the seismometers S S Thus, when the trains of seismic Waves arrive at the seismorneters S S electrical Waves generated by the seismometers are impressed upon the inputs of the corresponding recording channels H H Initiaily the first amplifier sections A A have low values of gain while the AVG amplifiers A A have high values of gain. As soon as seismic wave trains arrive at the respective seismometers the corresponding electrical waves generated by the seismorneters are amplified by the corresponding amplifier sections A and A", thereby producing large voltages at the outputs of the various amplifiers A, thus causing large deviations in the frequencies at the outputs of the respective fre quency modulated generators G. Suchlarge deviations as will become apparent hereinafter produce sharp first breal s. e

As early waves of the respective wave trains arrive at the seismometers S S the gains of the output sections A A are reduced at a rather rapid rate by virtue the action of the AVG loops, thus causing the outputs of the AVG amplifiers to attain outputs near their normal AVQ levels, subject to minor short-term variais close to theheads.

responding to the reference frequency of the generators.

tions corresponding to rapid changes in the amplitude of the seismic waves reflected or refracted from various subsurface formations. As seismic Waves continue to arrive at the seismometers S 8,; the gains of the input sections A A increase to some extent compensating for the gradual attenuation of the energy in the seismic wave trains. Generally, the time constants of the expander 110 are so set as to limit the input to the AVG amplifier sections A A to the range of signals in which AVC action is elfective. Consequently, except possibly for a large rapid increase in voltage occurring shortly after the first arrivals, the amplitudes of the waves supplied to the inputs of the frequency modulated generators G G are limited to a range in which the frequency modulation is linear, that is, to a range in which the deviation of frequency at the output of each frequency modulated generator G is proportional to the voltage impressed upon its input. Thus, while the seismic waves are being recorded, high fidelity frequency modulated tracks are produced in the respective zones Zg Z6 of the magnetic tape M as it travels along its path 12 during the next revolution of the tape.

The next time that the perforation p enters the gap between the contacts K of the detector switch, a positive pulse is applied to the grid circuit of the third sequence thyratron Y thereby operating the third sequence relay When relay R operates, sticking contacts K(9, 1) close, thereby energizing the blue STOP light L At the same time, contacts K(9, 3) open, de-energizing and restoring the channel relays R nel relays are restored, the magnetic heads D D are disconnected from the outputs of the recording channels H H thereby preventing any recording of any seismic waves subsequently received by the seismometers S S It will be understood that if such waves were recorded they would erase the waves previously recorded. The complete record so produced, continues to travel along its path 12.

Thereafter, at any time desired, the movement of the magnetic tape may be discontinued by opening the first motor switch E Also, thereafter at any time, the remaining relays R7 R may be de-energized and restored and the associated thyratrons Y Y may be de-energized by opening the ready switch E. It will be noted that when the ready switch E is now open, no change in the condition of the channel relays R R occurs, thereby holding the transducer heads D D disconnected from the output of the recording channels H H and connected to the inputs of the reproducing channels h h If the record produced is to be preserved, the motor m is stopped and the record replaced.

But if it is desired to produce a second record without preserving the record already made, the operation described above is repeated. Briefly, the ready switch E is closed and a charge of explosive E is detonated when the .FIRE light L is illuminated. Therefore, outputs simultaneous erase and recording is possible because the pulses applied to the heads saturate the portions of the record exposed to the magnetic fields of the heads.

Saturation of the magnetic tape by the fields produced by the heads is possible so long as the magnetic tape M It is thus apparent that the fact that no recording occurs while the splice P is in the transducing zone Z not only produces a record free of any disturbance from the splice P, but also produces a record which may be readily erased by subsequently recording operations.

It is to be noted that if the amplitudes ofthe outputs of the various amplifiers in 'the recording channels H are zero, nevertheless, a record is made cor- R When the chan-' In any event, the record made on the'magnetic tape M represents a faithful reproduction of the seismic waves received by the seismometers S 8,; by virtue of the fact that broad-band amplifiers A are employed in the recording channels H H These records may then be reproduced by the seismic wave reproducer 200 and re-recorded by the oscillograph O. In reproducing a magnetic tape record, the amplifiers A A in the reproducing channels h, 11 are adjusted in any desired fashion to emphasize seismic waves in any selected frequency band. Also, if desired, the amplifiers A Ag of the reproducing channels may be high-fidelity, broad-band amplifiers in order to produce accurate broadband seismograms of the waves actually recorded on the magnetic tape M. The method employed in reproducing the magnetic tapes and recording seismograms is described briefly below.

When a seismic record on a magnetic tape M is to be reproduced and re-recorded, the power switch E and the first motor switch E are closed to cause the magnetic tape M to move along its path 12. The record-reproducer switch E is left open, thereby disconnecting all of the channel relays R R from the power source so that these relays remain restored even though the sequence controller is operated. While it is not necessary to operate the sequence controller during reproduction of the records, nevertheless this may be done by simply closing the ready switch 13,. By virtue of the fact that switch E is open, the channel relays R R remain restored, thus assuring that the record will not be erased accidentally.

Thereafter the adjustable filters of the output amplifiers A' are set to filter out any undesired frequency components of the waves and to emphasize any components which it is desired to record. The various electronic units in the reproducing channels h are then energized preparatory to making a record.

Then the motor switch E is closed, energizing the camera motor m which drives the paper of the oscillograph 0. In practice, the switch E is closed at a time just after the splice P has passed the magnetic heads D D The passage of the splice P past the magnetic heads may be determined visually by watching the magnetic tape or by observing the movement of the beams of light reflected by the galvanometers onto the record paper, or by observing the indicator lights of the sequence controller. As soon as the camera motor In, is energized, it attains full speed quickly and the record paper moves past the beams of light reflected onto the paper from the galvanometers. The waves reproduced by the channels h h are recorded, forming a seismogram bearing traces that represent the signals that were recorded on the magnetic tape M. In practice, the oscillograph 0 may include a mechanism for recording timer lines on the seismogram to aid in correcting for any variations of motor speed before the time record from the magnetic tape becomes manifest. In any event, when the seismogram is made, seven traces are produced, each corresponding to the waves and other signals recorded on a corresponding track of the magnetic tape M.

One of the traces corresponding to the record made by recording the output of the auxiliary channel h bears 'a time break TB, an indication of the first break 'FB corresponding to the first wave that arrives at the uphole seismometer S and then a periodic waveform representing the signal impressed upon the magnetic tape M bythe timer 310. Each of the traces represents the record produced on the corresponding track upon which the output of the corresponding recording channel H has been impressed. V I a Though there may be a gap in the timer trace so produced, the fact that the camera motor is operating at a constant speed makes it possible to extrapolate to the time break and to the first break, or either, to determine the times at which they occurred relative to the times of g 13 occurre cc of various events indicated by the traces. The timer linesproduced by the oscillograph O itself aid in this extrapolation in case the camera motor has not reached full speed before the time break TB or uphole first break.

If desired, in order to check the operation of the recorder 161), the outputs of the various amplifiers A may be connected to the galvanometer elements through auxiliary circuits AX, as illustrated schematically in FIG. 6. With this connection, an oscillographic record may be made simultaneously with a magnetic tape record. Thereafter, by setting the adjustable filters of the amplifiers A to pass a broad frequency band, the magnetic record may be reproduced by means of the oscillograph O and compared with the record previously obtained.

The adjustable filters of the various amplifiers A' may be set at different values and corresponding records made by reproducing and recording signals from the magnetic tape.

While the apparatus described herein may be embodied in many different forms and may be employed in many other ways than that described, it will be clear that an improved system for recording and reproducing seismic records is provided by this invention. It is therefore to be understood that the invention is not limited to the specific embodiments thereof described and illustrated herein, but includes all forms thereof that come within the scope of the appended claims.

Iclaim:

1. In seismic prospecting apparatus:

an endless record'belt;

driving means for driving said belt along a predetermined path;

a plurality of transducers mounted in a transducing area adjacent said path in operative relationship with difierent longitudinal zones of said belt;

a plurality of seismic wave recording channels having a series of mutually spaced seismometers in their respective inputs;

a corresponding plurality of reproducing channels having their outputs connected to corresponding recording elements of a multiple-element oscillograph, said seismic wave recording channels having a broad pass band;

adjustable filtering means in said reproducing channels, said adjustable filtering means being adapted to selectively emphasize waves in various frequency bands of seismic waves previously recorded on said endless record belt; and

relay means for selectively connecting said transducers to the outputs of the corresponding recording channels or to the inputs of the corresponding reproducing channels.

2. in seismic prospecting apparatus:

an endless record belt;

driving means for driving said belt along a predetermined path;

a plurality of transducers mounted in a transducing area adjacent said path in operative relationship with di ferent longitudinal zones of said belt;

a plurality of seismic wave recording channels having a series of mutally spaced seismometers in their respective inputs, each of said recorder channels comprising a recording amplifier and a generator, the frequency of which is modulated in accordance with the output of the corresponding amplifier;

a corresponding plurality of reproducing channels having their outputs connected to corresponding recordin g elements of a multiple-element oscillograph, each of said reproducing channels comprising a demodulator and a reproducing amplifier;

and means for selectively connecting said transducing devices to the outputs of the corresponding recording channels or to the inputs of the corresponding reproducing channels.

3. Apparatus for recording and analyzing a seismic detector signal comprising means for generating a reference timing signal, a first recording medium, a second recording medium, means for recording said reference timing signal and said seismic detector signal on both said first and said second recording media, means for reproducing said detector signal and said timing signal from said first recording medium, means for modifying said reproduced seismic detector signal, and means for recording said modified seismic detector signal and said reproduced timing signal on said second recording medium, whereby said original seismic detector signal and said modified seismic detector signal are both recorded on said second recording medium and may be compared on the basis of a common timing signal.

4. Apparatus for recording and analyzing a seismic detector signal comprising means for generating a reference timing signal, a reproducible recording medium, a photographic recording medium, means for recording said reference timing signal and said seismic detector signal on both said reproducible recording medium and said photo gnaphic recording medium, means for reproducing said detector signal and said timing signal from said reproducible recording medium, means for modifying said reproduced seismic detector signal, and means for recording said modified seismic detector signal and said reproduced timing signal on said photographic recording medium, whereby said original seismic detector signal and said modified seismic detector signal are both recorded on said photographic recording medium and may be compared on the basis of a common timing signal.

5. Apparatus for recording and analyzing a seismic detector signal comprising means for generating a reference timing signal, a first recording medium, a second recording medium, means for recording said reference timing si nal and said seismic detector signal on both said first and second recording media, means for reproducing said detector signal and said timing signal from said first recording medium, means for modifying said reproduced seismic detector signal, and means for recording said modified seismic detector signal and said reproduced timing signal on said second recording medium.

6. Apparatus for recording and analyzing a seismic detector signal comprising means for generating a reference timing signal, a reproducible recording medium, a photographic recording medium, means for recording said reference timin signal and said seismic detector signal on both said reproducible recording medium and said photographic recording medium, means for reproducing said detector signal and said timing gnal from. said reproducible recording medium, means for modifying said reproduced seismic detector signal, and means for recording said modified seismic detector signal and said reproduced timing signal on said photographic recording medium.

7. Apparatus for recordingand analyzing a seismic detector signal comprising means for generating a reference timing signal, a first recording medium, a second recording medium, means for recording said reference timing signal simultaneously on both said first and second recording media, means for recording said seismic detectorsignal simultaneously on both said first and second recording media, means for reproducing said detector signal and said a ming signal that have been recorded on said first recording medium, means for modifying said reproduced seismic detector signal, and means for recording said modified seismic detector signal and said reproduced timing signal on said second recording medium.

8. Apparatus for recording and analyzing a plurality of related seismic detector signals comprising means for generatin a reference timing signal, a first recording medium, a second recording medium, means for recording said reference timing signal simultaneously on both said first and second recording media, means for recording said seismic detector signals simultaneously on both said first and second recording media and on separate tracks on each recording medium, means for reproducing said detector signals and said timing signal that have been recorded on said first recording medium, means for modifying said reproduced seismic detector signals, and means for recording said modified seismic detector signals and said reproduced timing signal on said second recording medium.

References Cited in the file of this patent 16 Woolf etv a1 July 8, 1941 Ridings Aug. 15, 1944 Petty Jan. 9, 1945 Begun June 19, 1945 Potter Oct. 21, 1947 Smith Apr. 6, 1948 Williams et al June 22, 1948 Hawkins Mar. 8, 1949 Shaper et a1. July 4, 1950 Begun et a1 Mar. 11, 1952 Lee et a1 Dec. 9, 1952 McCollum Oct. 16, 1956 Sharpe June 11, 1957 Loper Aug. 23, 1960 

3. APPARATUS FOR RECORDING AND ANALYZING A SEISMIC DETECTOR SIGNAL COMPRISING MEANS FOR GENERATING A REFERENCE TIMING SIGNAL, A FIRST RECORDING MEDIUM, A SECOND RECORDING MEDIUM, MEANS FOR RECORDING SAID REFERENCE TIMING SIGNAL AND SAID SEISMIC DETECTOR SIGNAL ON BOTH SAID FIRST AND SAID SECOND RECORDING MEDIA, MEANS FOR REPRODUCING SAID DETECTOR SIGNAL AND SAID TIMING SIGNAL FROM SAID FIRST RECORDING MEDIUM, MEANS FOR MODIFYING SAID REPRODUCED SEISMIC DETECTOR SIGNAL, AND MEANS FOR RECORDING SAID MODIFIED SEISMIC DETECTOR SIGNAL AND SAID REPRODUCED TIMING SIGNAL ON SAID SECOND RECORDING MEDIUM, WHEREBY SAID ORIGINAL SEISMIC DETECTOR SIGNAL AND SAID MODIFIED SEISMIC DETECTOR SIGNAL ARE BOTH RECORDED ON SAID SECOND RECORDING MEDIUM AND MAY BE COMPARED ON THE BASIS OF A COMMON TIMING SIGNAL. 